1. Field of the Invention
The present invention relates to an apparatus for forming a stack of folded printed products discharged from a printing machine and sequentially conveyed with the folds leading.
2. Description of the Prior Art
With the stack former H-500 of the firm "Fehr-Reist AG", Hinwil, Switzerland the printed product is moved on a support face or on the free face of a part stack already formed. On the thus moved product a perpendicular force is exerted to compress the part stack being built up. For exerting the perpendicular force two roller wheels run on the free end face of the part stack contacting it in a predetermined distance from the fold of the folded product, so that the fold of each product moves under the roller wheels and contacts then the stop means without being biased anymore. The contact positions of the two roller wheels have the same distance from the fold, while the distance from the fold is smaller than the distance from the trailing edge of the printing products so that the products being moved onto the part stack come into contact with the roller wheels just shortly before the stop means in order to avoid the frictional forces being induced by the perpendicular forces and hindering the movement of the product onto the part stack from becoming too high. The conveyor for the printing products does not overlap the free end face of the part stack, so that upon movement of the printing products onto the part stack the tangential forces can only be applied onto the portion of the printing product not yet overlapping the end face of the part stack. The small rollers of the roller wheels themselves can only apply very low tangential forces for the pushing of the product against the stop means.
The printing products being discharged from the folder of a printing machine in form of a product-by-product stream of overlapping products do not lie absolutely plane in the range of their folds, but build up in this range more or less. If now a plurality of printing products being fold up from paper sheets are stacked one upon the other for forming stacks the form of the stack being built up will substantially deviate from the form of a parallelepiped, especially the cross section of the part stack perpendicular with respect to the folds will become trapezoidal. Thus, for the build up of a stack it would be of advantage if for the movement of the printing products onto the stack as small as possible perpendicular forces for the compression of the folds are applied and in that for the adjustment of the stack to the form of a parallelepiped as high as possible perpendicular forces are applied.
Additionally this problem is rendered more difficult in that the friction characteristics between the printing products may vary to a considerable extent due to the differences in the size of the printed areas and the type of the used printing inks and due to the different stiffness of the paper used for the print. These characteristics cannot be considered to a sufficient extent with the known stack forming apparatus.